President Obama speaks at the Fox Theater in Oakland, Calif., on Monday. Saul Loeb/AFP/Getty Images hide caption

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Voter walk outside of a polling place at the First Baptist Church of Windermere, in Orlando, Fla., during the state's primary on Jan. 31. Matt Rourke/AP hide caption

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President Obama at a Democratic Party election fundraiser in Chicago on June 1. Jason Reed/Reuters/Landov hide caption

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Bill Clinton speaks with President Obama in the White House Briefing Room after a private meeting in the Oval Office in 2010. The former president has become Obama's highest-profile advocate this campaign season. Jim Watson/AFP/Getty Images hide caption

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President Obama is applauded after signing the health care overhaul during a ceremony in the White House on March 23, 2010. Then-Gov. Mitt Romney signs a Massachusetts health care overhaul at Faneuil Hall in Boston on April 12, 2006. Win McNamee/Boston Globe via Getty Images hide caption

itoggle caption Win McNamee/Boston Globe via Getty Images